Transition metal dichalcogenides (TMDs or TMDCs) have garnered much interest recently due to their weakly layered structures, allowing for mechanical exfoliation down to a single atomic layer. As such, it is pertinent to re-examine the bulk properties of these materials in order to completely understand and predict what is happening in the few-layered limit. A large majority of these systems were first investigated in the 1950s and 1960s. As such, many of the current growth methods rely on... Show moreTransition metal dichalcogenides (TMDs or TMDCs) have garnered much interest recently due to their weakly layered structures, allowing for mechanical exfoliation down to a single atomic layer. As such, it is pertinent to re-examine the bulk properties of these materials in order to completely understand and predict what is happening in the few-layered limit. A large majority of these systems were first investigated in the 1950s and 1960s. As such, many of the current growth methods rely on these reports, making new growth techniques for lowering defects of importance as well. In this thesis, both topics are taken into consideration and discussed, though the latter remains to be investigated in much more detail and should be the work of future research efforts. Orthorhombic MoTe₂ and its isostructural compound WTe₂ were recently claimed to belong to a new class (type II) of Weyl semimetals characterized by a linear touching between hole and electron Fermi surfaces in addition to nodal lines. These compounds have recently been shown to display very large non-saturating magnetoresistances which have been attributed to nearly perfectly compensated densities of electrons and holes. Here, we present a detailed study on the temperature and angular dependence of the Shubnikov-de-Haas (SdH) effect in the semi-metal WTe₂ and MoTe₂. In WTe₂, we observe four fundamental SdH frequencies and attribute them to spin-orbit split, electron- and hole-like, Fermi surface (FS) cross-sectional areas. Their angular dependence seems consistent with ellipsoidal FSs with volumes suggesting a modest excess in the density of electrons with respect to that of the holes. We show that density functional theory (DFT) calculations fail to correctly describe the FSs of WTe₂. When their cross-sectional areas are adjusted to reflect the experimental data, the resulting volumes of the electron/hole FSs obtained from the DFT calculations would imply a pronounced imbalance between the densities of electrons and holes. We find evidence for field-dependent Fermi surface cross-sectional areas by fitting the oscillatory component superimposed onto the magnetoresistivity signal to several Lifshitz-Kosevich components. We also observe a pronounced field-induced renormalization of the effective masses. Taken together, our observations suggest that the electronic structure of WTe₂ evolves with the magnetic field due to the Zeeman splitting. This evolution is likely to contribute to its pronounced magnetoresistivity. For β-MoTe2, high quality single-crystals were synthesized by flux in excess tellurium. We find that its superconducting transition temperature depends on disorder as quantified by the ratio between the room- and low-temperature resistivities, (residual resistivity ratio, RRR. Similar to WTe₂, its magnetoresistivity does not saturate at high magnetic fields and can easily surpass 10₂⁶%, with the superimposed Shubnikov de Haas oscillations revealing a non-trivial Berry phase of ≃pi. The geometry of the Fermi surface, as extracted from the quantum oscillations, is markedly distinct from the calculated one. A broad anomaly seen in the heat capacity and in the Hall-effect indicates that the crystallographic and the electronic structures evolve upon cooling below 100 K, likely explaining the discrepancy between these recent predictions and our experimental observations. In α-MoTe₂, grown at lower temperatures in vapor transport, we focus on few-layered crystals mechanically exfoliated onto a 270 nm thick SiO₂ layer. Previous reports found that the field-effect mobility of transition metal dichalcogenides TMDs tends to increase, reaching a maximum value when crystals are composed of approximately 10 atomic layers. We show that the overall performance of a MoTe₂ based field-effect transistor is comparable to similar devices based on MoS₂ or MoSe₂. But with an optical gap quite close in value to the one of Si and an enhanced spin-orbit interaction (since Te is a 5p element) suggests that this compound might be particularly suitable for optoelectronic applications in a complimentary range of wavelengths. In the case of MoSe₂, we show that multi-layered (~ 10 atomic layers) field-effect transistors can display ambipolar behavior at room temperature when using a standard combination of metals, Au on Ti, for all the electrical contacts. The 4.33 eV work function of Ti is closely matched by the electron affinity of bulk MoSe₂, 4.45 ± 0.11 eV. This implies that the Fermi level of Ti is very close to the bottom of the conduction band of MoSe₂, and therefore that one should expect a rather small Schottky barrier for electron conduction through the Ti:Au contacts. One extracts through Hall effect measurements, Hall mobilities in excess of 250 cm₂²/(V s) for both holes and electrons at room temperature. These values are remarkable, since they are comparable or higher than most values reported so far for transition metal dichalcogenides at room temperature, but are obtained without the use of high k-dielectrics such as HfO₂, doping, or of a particular combination of metals for the electrical contacts. Our results suggest that improvements in fabrication, and on the quality of the starting material (with a lower amount of defects) could make field-effect transistors based on few atomic layers of synthetic MoSe₂ excellent candidates for complementary logic electronics. Finally, we report on alloys of MoTe₂ and WTe₂, Mo₁₋xWxTe₂, grown by a chemical vapor transport process with the goal of obtaining a phase diagram with respect to doping and temperature. These crystals have been analyzed for composition via EDS and investigated through high resolution transmission electron microscopy, scanning tunneling microscopy (STM) and ARPES. Transmission electron microscopy images clearly show that through W substitution we are able to synthesize both 2H, trigonal prismatic, and Td, orthorhombic structures. As opposed to other reports, we find a phase transition from the 2H- to the Td-phase at 10 % W doping, much lower than expected. This structure is characterized by a linear arrangement of atoms, as opposed to a hexagonal pattern. In addition, through examination of monolayers via TEM, we find that the W disperses randomly amongst the crystal as opposed to forming grain boundaries. Given the crystallinity and quality of the material, mapping the phase diagram should be of relative ease, however this work is still ongoing. As such, the phase diagram has not yet been completed and remains unreported in this manuscript. Show less

Date Issued

2016

Identifier

FSU_2016SU_Rhodes_fsu_0071E_13414

Format

Thesis

Title

High Frequency Inductive Measurements of Organic Conductors with the Application of High Magnetic Fields and Low Temperatures.

Organic conductors are interesting to study due to their low dimensionality that leads to a number of competing low temperature ground states. Comprised of a number of different molecules that can be varied by the substitution of one atom for another, organic systems also provide a large number of similar compounds that lend themselves to comparison studies. Two such low-dimensional organic conductors, Per2[Pt(mnt)2] and (TMTSF)2ClO4, which are members of large families of compounds, are the... Show moreOrganic conductors are interesting to study due to their low dimensionality that leads to a number of competing low temperature ground states. Comprised of a number of different molecules that can be varied by the substitution of one atom for another, organic systems also provide a large number of similar compounds that lend themselves to comparison studies. Two such low-dimensional organic conductors, Per2[Pt(mnt)2] and (TMTSF)2ClO4, which are members of large families of compounds, are the topic of this dissertation. Both materials are considered quasi-one-dimensional and have a number of low temperature transitions, some of which can be studied via changes in the magnetic properties of the systems. The Per2[M(mnt)2] family of compounds provides a system for exploring the similarities and differences of the system's properties when the metal M has a localized spin (M = Pt, Ni, and Fe) versus when the metal is diamagnetic (M = Au, Cu, and Co). In the case of Per2[Pt(mnt)2] - one of the compounds of focus in this dissertation - the metallic perylene chains undergo a metal- insulator transition due to the formation of a charge density wave at Tc ~ 8 K, which also occurs in Per2[Au(mnt)2] at 12 K. However, unlike in the M = Au compound, an additional transition occurs in the M = Pt compound due to the localized Pt spins (S = 1/2) on the insulating Pt(mnt)2 chains - the spin chains of Per2[Pt(mnt)2] undergo a spin-Peierls transition at 8 K. One focus of the experimental work of this dissertation focuses on the magnetic properties of the spin chains in Per2[Pt(mnt)2], via inductive susceptibility measurements at temperatures down to 0.5 K and fields up to 60 T. The experimental results show a coupling of the spin-Peierls and charge density wave states below 8 K and 20 T, above which both states are suppressed. Further measurements show a second spin state transition occurs above 20 T that coincides with a field induced insulating state in the perylene chains. These results support a strong coupling between the charge density wave and spin-Peierls state even at high magnetic fields, which are discussed in the context of other experimental results and theories. Additionally, a simple model is developed to explore the possible mechanisms behind the coupling of the two segregated chains. The other experimental part of this dissertation focuses on one member of the (TMTSF)2X family of compounds, where the anion molecule (X) can have an octahedral symmetry (X = PF6, SbF6, AsF6) or a tetrahedral symmetry (X = ClO4, ReO4, BF4). All of these compounds undergo metal-insulator transitions with the formation of a spin density wave, which can be suppressed and replaced by a transition to a superconducting state with the application of pressure in all but the X = ClO4 compound. In (TMTSF)2ClO4, which is the other compound of focus in this dissertation, both the spin density wave state and the superconducting state can be realized at ambient pressure; however, the determination of the state is dependent on the rate at which the material is cooled through an anion ordering temperature. If the sample is cooled too quickly it remains disordered and the sample enters the spin density wave state; on the other hand, if it cools slowly and the anions are allowed to order, superconductivity is realized at 1.2 K. This superconducting state has been experimentally studied with a wide variety of experimental techniques and much is known about its properties. However, nanoparticles of (TMTSF)2ClO4 have recently been realized, which opens up a new avenue of research on this compound, since the bulk properties of a material are often modified when its size approaches the length scale of the ground state order parameter. As such, the experimental work on (TMTSF)2ClO4 in this dissertation focuses on the critical temperature and fields of the superconducting state of the nanoparticles using the same inductive susceptibility technique mentioned above. The experiments on an assembly of (TMTSF)2ClO4 nanoparticles show that the nanoparticles exhibit bulk-like properties similar to those of randomly oriented crystals of the parent compound; possible explanations for this observation and future plans are discussed in this context. Show less

This dissertation focuses on the transport properties of the high-temperature superconductor LSCO in high magnetic fields, particularly the longitudinal magnetoresistance around optimum doping. In this region of the phase diagram the longitudinal magnetoresistance appears to exhibit non-Fermi liquid magnetoresisitance. Additionally, high-field magnetotransport in LBCO is included, which revealed a layered, phase-decoupled superconducting state for x = 0.095.

At low temperature (T) and weak magnetic field (B), two dimensional electron systems (2DES) can exhibit strong 1/B-periodic resistance oscillations on application of sufficiently strong microwave radiation. These oscillations are known as microwave induced resistance oscillations (MIROs), MIROs appearing near cyclotron resonance (CR) and its harmonics involve single photon processes and are called integer MIROs while the oscillations near CR subharmonics require multiphoton processes and are... Show moreAt low temperature (T) and weak magnetic field (B), two dimensional electron systems (2DES) can exhibit strong 1/B-periodic resistance oscillations on application of sufficiently strong microwave radiation. These oscillations are known as microwave induced resistance oscillations (MIROs), MIROs appearing near cyclotron resonance (CR) and its harmonics involve single photon processes and are called integer MIROs while the oscillations near CR subharmonics require multiphoton processes and are called fractional MIROs. Similar strong 1/B periodic resistance oscillations can occur due to strong dc current, and are known as Hall-field resistance oscillations (HIROs). Oscillations also occur for a combination of microwave radiation and strong dc current. In one prominent theory of MIROs, known as the displacement model , electrons make impurity-assisted transitions into higher or lower Landau levels by absorbing or emitting one or more (N) photons. In the presence of combined strong dc current and microwave radiation, electrons make transitions between Landau levels by absorbing or emitting photons followed by a space transition along the applied dc bias. The object of the dissertation is to explore how the different resistance oscillations are affected by strong microwave radiation when multiphoton processes are relevant. We used a coplanar waveguide (CPW) structure deposited on the sample, as opposed to simply placing the sample near the termination of a waveguide as is more the usual practice in this field. The CPW allows us to estimate the AC electric field (E_{AC}) at the sample. In much of the work presented in this thesis we find that higher $N$ processes supersede the competing lower N processes as microwave power is increased. We show this in the presence and in the absence of a strong dc electric field. Finally, we look at the temperature evolution of fractional MIROs to compare the origin of the fractional MIROs with that of integer MIROs. Show less

Transition metal oxides and chalcogenides have been the major focus of studies in condensed matter physics. The complexity of the system, involving spin and orbital effects, as well as lattice degree of freedom, makes them intriguing subjects not only because of these individual effects, but also the effects due to the interaction among them. In AB2X4 materials (A = Mn2+, Co2+, Fe2+; B = V3+, Cr3+; X = O2-, S2-) which crystallize in spinel structure (space group F d -3 m), these effects and... Show moreTransition metal oxides and chalcogenides have been the major focus of studies in condensed matter physics. The complexity of the system, involving spin and orbital effects, as well as lattice degree of freedom, makes them intriguing subjects not only because of these individual effects, but also the effects due to the interaction among them. In AB2X4 materials (A = Mn2+, Co2+, Fe2+; B = V3+, Cr3+; X = O2-, S2-) which crystallize in spinel structure (space group F d -3 m), these effects and their interactions manifest in their transport properties, magnetic ordering, itinerant electron magnetism, structural distortion, and geometrical frustration effect due to the antiferromagnetically coupled B-sites. These effects are dependent on the distance between the interacting cations, which can be varied by chemical substitution or pressure. The main objective of this dissertation is to study the physical properties of Mott-insulator spinels in approaching their critical inter-cationic distances where an insulator-metal transition occurs. Studying the insulator-metal transition in Mott insulators is important in advancing our understanding, especially in the field of fundamental physics and materials engineering, on the intricate relationships between the transport and magnetic properties and the emergence of new behaviors that arise from such properties in these materials. In this dissertation, the behavior of the physical properties of Mn1-xCoxV2O4, AV2O4 (A = Cd, Mg, Zn), and the transport properties of FeCr2S4 in approaching the insulator-metal transition are reported. Mn1-xCoxV2O4, AV2O4, and FeCr2S4 are chosen for this study due to their dominant V-V or Cr-Cr interactions, which are responsible for their transport properties. In Mn1-xCoxV2O4, the vanadium-vanadium distance is varied by means of chemical pressure (chemical substitution) to bring the system closer to the itinerant electron limit given by the critical V-V distance of 2.94 Å. In Mn1-xCoxV2O4, the structural distortion temperature and transport activation energy decreases with decreasing V-V distance, while the magnetic ordering temperature increases. The results of the transport and structural studies are in agreement with the critical V-V distance scenario of electronic delocalization. Next, a comparative structural study on AV2O4 with non-magnetic A-site ions (A = Cd, Mg, Zn) and Mn1-xCoxV2O4 is also reported. The study indicates that while the V-V interactions are dominant, the A-site ions and their magnetism produce a considerable effect on the passage from the localized to delocalized electron limit. This is proven by the two paths that emerge in the V-V distance dependence of the transport and structural properties where one path includes only the AV2O4, whereas the other includes only Mn1-xCoxV2O4. The transport property of FeCr2S4 under high pressure was also studied. Due to the t2g electronic configuration of Cr3+, the Cr-Cr interaction is also dominant. A high pressure measurement using a cubic anvil press up to 8 GPa was performed to induce an insulator-metal transition. The decrease in the Cr-Cr distance with increasing hydrostatic pressure was confirmed by x-ray diffraction measurements. The Bloch parameter of FeCr2S4 was found to be -2.4, which suggests that FeCr2S4 lies in the localized regime. The high pressure transport measurement on FeCr2S4 shows a decrease in the activation energy and an increase in the magnetic transition temperature with increasing hydrostatic pressure. An insulator to metal transition was observed at a pressure of 7.5 GPa with a possible onset at 7 GPa, at which the Cr-Cr distance is 3.44 Å. In the case of Cr-oxides, it was predicted that the critical Cr-Cr distance is 2.84 Å, but it should be higher for a less electronegative anion. Therefore, the difference in the anion species is responsible for the difference of 0.6 Å between the critical Cr-Cr distance in oxides and the actual Cr-Cr distance where the insulator-metal transition occurs. The insulator-metal transition is followed by a structural transformation at P = 8 GPa. Show less

Superfluid 4He (He II) has been widely used as a coolant material in many engineering applications. Its unique heat transfer mode is the so-called thermal counterflow. The study of thermal counterflow will contribute to the design of He II based cooling devices and our understanding of quantum turbulence. However, due to the lack of effective visualization and velocimetry techniques, studying the fluid dynamics in superfluid 4He is very challenging. In this dissertation, we discussed the... Show moreSuperfluid 4He (He II) has been widely used as a coolant material in many engineering applications. Its unique heat transfer mode is the so-called thermal counterflow. The study of thermal counterflow will contribute to the design of He II based cooling devices and our understanding of quantum turbulence. However, due to the lack of effective visualization and velocimetry techniques, studying the fluid dynamics in superfluid 4He is very challenging. In this dissertation, we discussed the development of a novel flow-visualization technique in He II based on the generation and imaging of thin lines of metastable tracer molecules. These molecular tracers are created via femtosecond-laser field-ionization of helium atoms and can be imaged using a laser-induced fluorescence technique. In steady state thermal counterflow measurement, we demonstrated that such tracer molecules are entrained by the normal fluid component. We revealed for the first time a laminar to turbulent transition in the normal fluid component. We found that the profile of the normal fluid in the laminar flow can exhibit quite different velocity profile compared to the laminar Poiseuille profile of classical fluid in a channel. In the turbulent flow state, the turbulence intensity is found to be much higher than that in classical channel flow. This turbulence intensity appears to depend primarily on temperature. We also found that the form of the second order transverse structure function deviates more strongly from that found in classical turbulence as the steady state heat flux increases, suggesting novel energy spectrum. In decaying counterflow turbulence, we studied the normal fluid flow via flow visualization and measured the quantized vortex line density using 2nd sound attenuation. Comparing the decay behavior of both fluids, we were able to produce a theoretical model to explain the puzzling decay behavior of the vortices. We were also able to determine the effective kinematic viscosity in a wide temperature range. Some preliminary results in the study of decaying grid turbulence were obtained, which allows us to examine the intermittent behavior of superfluid turbulence. Show less

Date Issued

2017

Identifier

FSU_SUMMER2017_Gao_fsu_0071E_13828

Format

Thesis

Title

Electronic Tuning in the Hidden Order Compound URu2Si2 through Si → P Substitution.

Crystalline materials that include 4f- and 5f-electron elements frequently exhibit a variety of intriguing phenomena including spin and charge orderings, spin and valence fluctuations, heavy fermion behavior, breakdown of Fermi liquid behavior, and unconventional superconductivity. [5, 6, 7, 8, 9, 10, 11, 12, 13] Amongst such materials, the Kondo lattice system URu2Si2 stands out as being particularly unusual. [14, 15, 16] While at high temperature it exhibits behavior that is typical for an... Show moreCrystalline materials that include 4f- and 5f-electron elements frequently exhibit a variety of intriguing phenomena including spin and charge orderings, spin and valence fluctuations, heavy fermion behavior, breakdown of Fermi liquid behavior, and unconventional superconductivity. [5, 6, 7, 8, 9, 10, 11, 12, 13] Amongst such materials, the Kondo lattice system URu2Si2 stands out as being particularly unusual. [14, 15, 16] While at high temperature it exhibits behavior that is typical for an f-electron lattice immersed in a sea of conduction electrons, at T0 = 17:5 K there is a second order phase transition that is followed by unconventional superconductivity near Tc 1:5 K. [15] Despite three decades of work, the order parameter for the transition at T0 remains unknown and hence, it has been named "hidden order". There have been a multitude of experimental attempts to unravel hidden order, mainly through tuning of the electronic state via pressure, applied magnetic field, and chemical substitution. [17, 18] While these strategies reveal interesting phase diagrams, a longstanding challenge is that any such approach explores the phase space along an unknown vector: i.e., many different factors are affected. To address this issue, we developed a new organizational map for the U-based ThCr2Si2-type compounds that are related to URu2Si2 and thus guided, we explored a new chemical tuning axis: Si -> P. Our studies were enabled by the development of a new molten metal crystal growth method for URu2Si2 which produces high quality single crystals and allows us to introduce high vapor pressure elements, such as phosphorous. [19, 20] Si -> P tuning reveals that while the high temperature Kondo lattice behavior is robust, the low temperature phenomena are remarkably sensitive to electronic tuning. [21, 22] In the URu2Si2-xPx phase diagram we find that while hidden order is monotonically suppressed and destroyed for x < 0.035, the superconducting strength evolves non-monotonically with a maximum near x = 0.01 and that superconductivity is destroyed near x = 0.028. For 0.03 < x < 0.26 there is a region with Kondo coherence but no ordered state. Antiferromagnetism abruptly appears for x = 0.26. This phase diagram differs significantly from those produced by most other tuning strategies in URu2Si2, including applied pressure, and isoelectronic chemical substitution (i.e. Ru -> Fe and Os), where hidden order and magnetism share a common phase boundary. [2, 23, 24] We discuss implications for understanding hidden order, its relationship to magnetism, and prospects for uncovering novel sibling electronic states. Show less

This dissertation is focused on electrical spin injection and detection at the nanoscale dimensions that semiconductor nanowires offer. Semiconductor spintronics is the natural extension of metallic spintronics for applications in semiconductor industry. After the tremendous impact of the giant magnetoresistance effect (GMR) in hard disk read heads, semiconductor spintronics has been thought as the key ingredient for the realization of spin field-effect transistors (Spin-FETs). The advantages... Show moreThis dissertation is focused on electrical spin injection and detection at the nanoscale dimensions that semiconductor nanowires offer. Semiconductor spintronics is the natural extension of metallic spintronics for applications in semiconductor industry. After the tremendous impact of the giant magnetoresistance effect (GMR) in hard disk read heads, semiconductor spintronics has been thought as the key ingredient for the realization of spin field-effect transistors (Spin-FETs). The advantages of spintronic devices would include non-volatility, enhanced data processing speeds, decreased electric power consumption and facilitation of quantum computation. The primary goal of this research is to study spin dynamics and spin-polarized transport in semiconductor nanowire (NW) channels, specifically in phosphorus (P) doped silicon (Si) nanowires (NWs). The interest in one-dimensional (1D) nanoscopic devices is driven by the rich spin-dependent physics quantum confinement engenders, and the eventual miniaturization of the spintronic devices down to nanoscales. One of the most important aspects to achieve efficient spin injection from a ferromagnet (FM) into a semiconductor (SC) is the interface between the two materials. This study is focused primarily on this effect and how it can be tuned. In this work, we peform systematic spin transport measurements on a unique type of P-doped Si NWs which exhibit an inherent doping gradient along the axial direction. On a single NW, we place a series of FM electrodes, which form contacts that evolve from Ohmic-like to Schottky barriers of increasing heights and widths due to the pronounced doping gradient. This facilitates rigorous investigation of the dependence of the spin signal on the nature of the FM/SC interface. The advantage of using a single NW to study the afformentioned effects is that possible complications during the fabrication process are minimized compared to experiments that use multiple different devices to perform such experiments. 2-terminal (2T), nonlocal 4-terminal (4T) and 3-terminal (3T) spin valve (SV) measurements using different configurations of FM electrodes were performed on the Si NWs. In addition, 3T and nonlocal 4T Hanle measurements were performed. The collected data reveal distinct correlations between the spin signals and the injector and detector interfacial properties. These results were possible due to the unique inhomogeneous doping profile of our Si NWs. This study reveals a distinct correlation between the spin signals and the FM/Si NW injector interfacial properties. Specifically, we observe a decreasing injected current spin polarization due to diminishing contribution of the d-electrons, thus the necessary tunneling contact for efficient spin injection and its properties are being investigated and analyzed. The results demonstrate that there is an optimal window of interface resistance parameters for maximum injected current spin polarization. In addition, they suggest a new approach for maximizing the spin signals by making devices with asymmetric interfaces. To the best of our knowledge, this is the first report of electrical spin injection in SC channels with asymmetric interfaces. Show less

This dissertation presents studies on mononuclear single molecule magnets (SMMs) with magnetic properties arising from transition metal ions in trigonal bipyramidal (TBP) coordination environments. We use both experimental and theoretical methods to elucidate the effects of coordination geometry on the magnetic anisotropy of a SMM. The role of an axial magnetic anisotropy is to pin the magnetic moment of the metal ion in one of two preferred orientations, either parallel or anti-parallel to... Show moreThis dissertation presents studies on mononuclear single molecule magnets (SMMs) with magnetic properties arising from transition metal ions in trigonal bipyramidal (TBP) coordination environments. We use both experimental and theoretical methods to elucidate the effects of coordination geometry on the magnetic anisotropy of a SMM. The role of an axial magnetic anisotropy is to pin the magnetic moment of the metal ion in one of two preferred orientations, either parallel or anti-parallel to the magnetic easy-axis. For transition metals, maximization of the axial magnetic anisotropy requires stabilization of an unquenched orbital moment that can couple to a ligand field. SMMs with giant magnetic anisotropy play an important role both in terms of fundamental scientific reasons and potential application in information technologies. Thus the studies presented in this dissertation attempt to explore some of the interesting physics in these compounds. The presence of orbitally degenerate states and unquenched orbital momentum pushes the limits of spin-only model. To overcome this limitation, we propose a phenomenological spin-orbit model based on point charge approximation with the goal to investigate orbitally degenerate mononuclear compounds. As an application of our model, we consider two test compounds : Iron(II) and Nickel(II) ions in trigonal bipyramidal (TBP) environments, where we find that the high symmetry configuration supports a large magnetic anisotropy in the absence of Jahn-Teller distortion. The motivation for our phenomenological model stemmed from our detailed EPR measurements performed on a mononuclear Nickel(II) SMM in a TBP environment that revealed an unprecedented magnetic anisotropy, reaching the limits of applicability of the familiar spin-only description. The axial anisotropy estimated for this complex was found to the be the largest so far for a mononuclear Nickel(II) complex; and, importantly, only a very small degree of axial symmetry breaking was detected. This was most likely considered to be due to the unquenched orbital moment in the ground states of the Nickel(II) ion. To further confirm this prediction we performed theoretical studies of the SMM using the phenomenological spin-orbit model. This study showed the suppression of Jahn-Teller effects in trigonal bipyramidal Nickel(II) complex because of rigid, bulky axial ligands. To further understand the effects of using bulky ligands in TBP coordination environments we performed experimental and theoretical studies on a mononuclear Iron(II) SMM. Although the ground states of this complex is also orbitally degenerate, our investigation showed reduced axial magnetic anisotropy compared to Nickel(II) with a very small transverse component. Our phenomenological investigation of the ground states revealed that the magnitude of the first order contribution is strongly dependent on the bond angles, and the spin-orbit coupling constant also plays a significant role in achieving large magnetic anisotropy. Finally, we also explore the effects of different ligand types in Cobalt(II) mononuclear complexes in TBP coordination environments. In these Kramers systems we and that the combination of a 3-fold symmetric ligand and a trigonal space group gives rise to an increase in the easy-plane magnetic anisotropy, while keeping the rhombicity of the system close to zero. This is particularly interesting for quantum information processing, especially in relation to molecules with a large spin ground state characterized by a large easy-plane anisotropy. Show less

Date Issued

2018

Identifier

2018_Su_Bhaskaran_fsu_0071E_14700

Format

Thesis

Title

Magnetocaloric Effect in CeCoIn5 Utilizing a Miniature Cell for High Magnetic Fields and Low Temperatures.

Creator

Bernheisel, Ashley, Department of Physics

Abstract/Description

We studied the relationship between superconductivity and magnetism in the heavy fermion CeCoIn5 using a miniature cell in a top loading dilution refrigerator with a 16 T magnet (XCF – Xtreme Conditions Fridge). The construction and measurement techniques for a miniature cell are discussed. We were able to observe a first order superconducting phase transition with an upper critical field Hc2 at roughly 0.2 K. It changed position and shape with angle and magnetic field sweep rate because of... Show moreWe studied the relationship between superconductivity and magnetism in the heavy fermion CeCoIn5 using a miniature cell in a top loading dilution refrigerator with a 16 T magnet (XCF – Xtreme Conditions Fridge). The construction and measurement techniques for a miniature cell are discussed. We were able to observe a first order superconducting phase transition with an upper critical field Hc2 at roughly 0.2 K. It changed position and shape with angle and magnetic field sweep rate because of CeCoIn5's anisotropic crystal structure and small heat exchanges through the wires used in the calorimeter. Show less

Date Issued

2014

Identifier

FSU_migr_uhm-0293

Format

Thesis

Title

Charge density waves and superconductivity in alpha-uranium.

Creator

VanGennep, Derrick, Department of Physics

Abstract/Description

We have measured the electrical resistivity and magnetoresistance of several α-uranium single crystals under pressure. The residual resistivity ratios (RRRs) of these samples ranged from 158 to 265 and the pressure dependence is discussed. Superconductivity was observed at temperatures varying from approximately 20 mK at ambient pressure to 3 K at 16 kBar and critical fields were observed up to 0.5 T at 16 kBar. Sharp features were seen at each of the three charge density wave (CDW)... Show moreWe have measured the electrical resistivity and magnetoresistance of several α-uranium single crystals under pressure. The residual resistivity ratios (RRRs) of these samples ranged from 158 to 265 and the pressure dependence is discussed. Superconductivity was observed at temperatures varying from approximately 20 mK at ambient pressure to 3 K at 16 kBar and critical fields were observed up to 0.5 T at 16 kBar. Sharp features were seen at each of the three charge density wave (CDW) transitions starting near 40 K. The magnetic field and pressure dependences of the CDWs as well as superconductivity are presented and discussed. Show less

The purpose of this thesis is to present the results of how the preparation of a sample of Uranium-238 changes its crystal quality. First this thesis will explain the process of etching Uranium and what this does to a crystal batch. This thesis will then explore the process of "cutting" Uranium with an Electric Discharge Machine (EDM) and how this negatively affects the crystal. It will then delve into the effects of Jackson-polishing and how it improves crystal quality. The largest portion... Show moreThe purpose of this thesis is to present the results of how the preparation of a sample of Uranium-238 changes its crystal quality. First this thesis will explain the process of etching Uranium and what this does to a crystal batch. This thesis will then explore the process of "cutting" Uranium with an Electric Discharge Machine (EDM) and how this negatively affects the crystal. It will then delve into the effects of Jackson-polishing and how it improves crystal quality. The largest portion of this thesis will concentrate on the annealing of a crystal. It will explain what annealing is and why it too improves crystal quality. It will present the process used and data found concerning the melting point found for this particular batch of Uranium. It will then explore a range of temperatures and durations of anneals that either do or do not produce an improvement in crystal quality. It will also argue that certain shapes of crystals are affected differently by annealing than others. It will then provide a recommended path heading forward. Show less

Date Issued

2014

Identifier

FSU_migr_uhm-0295

Format

Thesis

Title

Optimization of Local Annealing Processes in GaAs/AlGaAs Heterostructures for Spintronic Applications.

Creator

Byrne, April M., Department of Physics

Abstract/Description

This thesis presents work on optimizing a local annealing process for creating Ohmic contacts to a 2D electron gas (2DEG) within a GaAs/AlGaAs heterostructure. The optimization of this process will allow for fabrication of Hall devices to be used in the study of magnetic properties in quantum dots. To confirm good Ohmic contacts had been made to the 2DEG, data including IV characteristics was collected to show a linear relationship between current and voltage. Also, in order to preserve the... Show moreThis thesis presents work on optimizing a local annealing process for creating Ohmic contacts to a 2D electron gas (2DEG) within a GaAs/AlGaAs heterostructure. The optimization of this process will allow for fabrication of Hall devices to be used in the study of magnetic properties in quantum dots. To confirm good Ohmic contacts had been made to the 2DEG, data including IV characteristics was collected to show a linear relationship between current and voltage. Also, in order to preserve the sample and its magnetic properties, temperature was measured near the contact to confirm that the contacts were heated to about 415°C with the quantum dots at a temperature between 200°C and 250°C. Show less

The discovery of graphene marked a turning point in research and interest towards 2 -D materials. Among them, Transition Metal Dichalcogenides (TMDs) and Metal Monochalcogenides (MM) have seen an upturn in interest owing to their versatile properties. Although, they have been studied for many years in bulk form, recent advances in nano-technology enabled new opportunities to study the role of atomically thin materials. In recent years much work has been dedicated to development of their... Show moreThe discovery of graphene marked a turning point in research and interest towards 2 -D materials. Among them, Transition Metal Dichalcogenides (TMDs) and Metal Monochalcogenides (MM) have seen an upturn in interest owing to their versatile properties. Although, they have been studied for many years in bulk form, recent advances in nano-technology enabled new opportunities to study the role of atomically thin materials. In recent years much work has been dedicated to development of their application for the next generation of electronic and optoelectronic devices, and we are witnessing the dawn of the exploration of their properties. In Chapter 1 a brief introduction of highlighted properties of the newly emerged 2 -D materials and their heterostructures is provided. Chapter 2 focuses on field-effect transistor response of few atomic layers of MoSe2, MoTe2 and WSe2. In contrast to previous reports on MoSe2 FETs electrically contacted with Ni, MoSe2 FETs electrically contacted with Ti display ambipolar behavior with current ON to OFF ratios up to 10^6 for both hole and electron channels when applying a small excitation voltage. For both channels the Hall effect indicates Hall mobilities H = 250 cm^2/V.s. Our MoTe2 field-effect transistors are observed to be hole-doped, displaying ON/OFF ratios surpassing 10^6 and typical subthreshold swings of ~140mV per decade. Both field-effect and Hall mobilities indicate maximum values approaching or surpassing 10 cm^2/V.s, which are comparable to figures previously reported for single or bilayer MoS2 and/or for MoSe2 exfoliated onto SiO2 at room temperature and without the use of dielectric engineering. Temperature dependent comparison between field-effect and Hall mobilities in field effect transistors based on few-layered WSe2 exfoliated onto SiO2 is also reported. We observe maximum hole mobilities approaching 350 cm^2/V.s at T = 300 K. The hole Hall mobility reaches a maximum value of 650 cm^2/V.s as T is lowered below ~150 K, indicating that insofar WSe2- based field-effect transistors (FETs) display the largest Hall mobilities among the transition metal dichalcogenides. Chapter 3 evaluates electrostatically gated p-n junctions based on MoSe2 and the photovoltaic response of electrostatically generated p-n junctions composed of approximately 10 atomic layers of MoSe2 stacked onto dielectric h-BN is presented. In addition to ideal diode-like response, we find that these junctions can yield photovoltaic effciencies exceeding 14% under standard solar simulator spectrum with fill factors values of about 70 %. Chapter 4 presents electrical and optical characterization of monolayer and bilayer lateral heterostructures of MoS2-WS2 and MoSe2-WSe2, grown by a one-pot chemical vapor deposition (CVD) synthesis approach, using a single heterogeneous solid source, a newly developed CVD growth method that eliminates the need for the exchange of multiple sources which leads to sample air exposure. The structures show a diode like response which is enhanced under optical illumination. Additionally, bilayer lateral heterostructures exhibit a clear photovoltaic response to optical excitation. Show less

Date Issued

2018

Identifier

2018_Sp_Memaran_fsu_0071E_14363

Format

Thesis

Title

High Magnetic Field Studies of Doped Plutonium and Uranium Based Superconductors.

Two heavy Fermion superconducting compounds, URu₂Si₂ and PuRhIn₅, were investigated by the techniques of chemical substitution and application of high magnetic fields. These materials are particularity interesting for their unique electronic and ordered state behavior (hidden order and superconducting phases) and proximity to magnetism. It is thought the superconductivity of these materials is unconventional and that they exhibit some features that are associated with quantum criticality,... Show moreTwo heavy Fermion superconducting compounds, URu₂Si₂ and PuRhIn₅, were investigated by the techniques of chemical substitution and application of high magnetic fields. These materials are particularity interesting for their unique electronic and ordered state behavior (hidden order and superconducting phases) and proximity to magnetism. It is thought the superconductivity of these materials is unconventional and that they exhibit some features that are associated with quantum criticality, especially for PuRhIn₅. In earlier work URu₂Si₂ was doped with the non-isoelectric element phosphorus to produce the doping series URu₂Si₂₋ₓPₓ. During the current study, single crystals of the series were placed in high pulsed magnetic fields (up to 65T) and the evolution of the field induced phases was observed. The parent compound exhibits five unique phases in field as does the doped series up to approximately x=0.3. At this concentration and at zero field the hidden order phase is destroyed and any higher doping exhibit no ordered ground state. Over this x-range there is only one field induced state. Further increasing x (x > 0.26) pushes the system into an antiferromagnetic ground state, which has some high field ordering but at a higher magnetic fields than the lower doped compounds. This behavior is similar to the effect of Co, Rh and Ir substitution, which are also non-isoelectronic dopants that add electrons. This is in contrast to isoelectric doping (using Fe or Os) in which produce effects in the material similar to applied pressure. From this, it appears that the effects of non-isoelectric dopants might be attributed to band filling. The hidden order state of the parent compound URu₂Si₂ was also investigated with an optical magnetostriction technique in high magnetic fields. A transition from a quadratic to linear field response is seen in the signal while still in the hidden order state. This behavior is unusual and possible explanations include partial polarization of the Fermi surface and quadrupolar interactions. The Pu based superconductor PuRhIn₅ was doped with Cd and placed in high magnetic fields. Pu is both a radiological and toxicity hazard. As a result, a significant part of this project was spent controlling these hazards while enabling measurements. The phase diagram of PuRh(In₁₋ₓCdₓ)₅ in the T-x-H phase diagram was mapped and the optical magnetostriction technique was applied to the parent compound. From this data the electronic Grüneisen ratio was determined. Applying scaling arguments it was shown that the Grüneisen data is consistent with proximity to a quantum critical point, which is though to figure heavily in unconventional superconducting systems. Show less

Complex physical phenomena, such as superconductivity, colossal magnetoresistance (CMR) effect, multi-ferroics, metal-insulator transition, quantum phase transition, etc. in strongly correlated materials have been enduring topics in condensed matter physics. The overarching theme of this dissertation is the study of transport and electronic states in emergent phases with distinct magnetic and electronic properties in strongly correlated materials in thin film forms. At first, we have... Show moreComplex physical phenomena, such as superconductivity, colossal magnetoresistance (CMR) effect, multi-ferroics, metal-insulator transition, quantum phase transition, etc. in strongly correlated materials have been enduring topics in condensed matter physics. The overarching theme of this dissertation is the study of transport and electronic states in emergent phases with distinct magnetic and electronic properties in strongly correlated materials in thin film forms. At first, we have investigated anisotropic electronic phase separation (EPS) of optimally doped La2/3Ca1/3MnO3 (LCMO) thin films under various degrees of anisotropic strain by static magnetotransport and dynamic relaxation measurements. Distinct from the prototype perovskite manganite LPCMO with well-known micrometer scale EPS, the bulk optimally doped LCMO does not exhibit the large-scale EPS near a transition from paramagnetic insulating phase (PMI) to ferromagnetic metallic (FMM) phase at a high temperature. Through epitaxial growth of LCMO thin films on NGO (001) substrates and post-growth annealing, an antiferromagnetic insulating phase is induced in the FMM ground state and results in a large-scale EPS of coexisting AFI and FMM phases below the bulk metal insulator transition (MIT). Substantial resistivity anisotropies along the two orthogonal in-plane directions in the EPS region were experimentally probed by static temperature and magnetic field dependent resistivity measurements. More strikingly, with increasing annealing time, resistivity along the tensile-strained [010] direction becomes progressively larger than that along the compressive-strained [100] direction in the EPS region. The enhanced resistivity anisotropy suggests that the EPS is characterized by phase-separated FMM entities with a preferred orientation along [100] direction, possibly due to the deformation and rotation of the MnO6 octahedra under the enhanced anisotropic strain via the post-growth annealing. Furthermore, the EPS was found to exhibit glass-like behavior. The resistivity measured at fixed temperatures relaxes logarithmically over a long period of time. The relaxation behavior also shows a coherent enhancement with increasing annealing time. By fitting the relaxation data to a phenomenological model, the fitted parameters, resistive viscosity and characteristic relaxation time were found to evolve with temperature, showing a close correlation with the static measurements in the EPS states. In another project, we have investigated the superconductor-insulator quantum phase transitions tuned by disorder (d), magnetic impurity (MI) and magnetic field (B) in ultrathin Pb films by electrical transport measurement and single electron tunneling spectroscopy. In the past decade, the investigation of SITs in homogeneous thin films by transport measurement from our group has provided valuable insights to the mechanisms of SITs. There are two main theoretical models to explain SITs. The first one emphasizes that a transition from a superconducting state to a fermionic insulator without the existence of the superconducting order parameter, the formation of Cooper pairs is completely suppressed at the transition. The other one calls for a bosonic insulator with localized Cooper pairs. d-tuned and MI-tuned SITs well fit the fermionic framework, and both share common transport features, such as a sharp resistive transition to the superconducting state, a well-defined phase boundary, and a weakly insulating state near the phase boundary. While B-tuned SITs are a canonical example of the bosonic model. The resistive transition to the superconducting state is broadened by an application of magnetic field. Rather than a clear phase boundary near the transition, emerged resistive reentrance and double reentrance indicate phase fluctuation of the superconducting parameter is the main driving force for the transition, suggesting the survival of Cooper pairs in the insulating phase. Electron tunneling spectroscopy has been proposed to directly probe the existence and evolution of the superconductivity in these SITs. For B-tuned SITs, the existence of Cooper pairs is supposed to be detected even in the global insulating phase of thin films. More importantly, the approach also allows us to compare the evolutions of the normal state density of state among these SITs, particularly for d-tuned and MI-tuned SITs. Transport results show that MI has little influence on the normal state sheet resistance near the transition, while increasing disorder gradually raises the normal state sheet resistance. These observations suggest that the normal state density of states behaves differently in the two transitions. The experimental setup is a dilution refrigerator incorporated with in situ quench condensation, electrical measurement, and sample rotation, enabling us to achieve and tune SITs in the same sample by different parameters, and systematically check and compare the evolution of the density of states in the SITs. Up to now, we have performed transport and tunneling measurements for d-tuned SITs in homogeneous Pb films in a 4He quench probe and the modified dilution refrigerator. The transport results are consistent with previous experiments from our group. Increasing disorder leads to a SIT characterized with a sharp resistive transition to a zero resistance state, a well-defined phase boundary, and a gradual reduction of the superconducting critical temperature. The preliminary tunneling testing in the quench probe successfully reveals the suppression of the superconducting energy gap and the normal state density of state by the increasing disorder. In the modified dilution refrigerator, we still observed a concomitant suppression of the normal state density of states. Unfortunately, we were not able to reproduce valid tunneling spectra to study the evolution of the superconducting energy gap near the Fermi level. Possible reasons for the unsatisfying tunneling results are discussed at the end. Show less

This dissertation presents the recent developments and experiments performed using the third generation femtosecond electron diffractometer in Professor Jianming Cao's group as well as experiments performed using the previous second generation diffractometer now located at Shanghai Jiao Tong University. Two techniques of ultrafast electron diffraction (UED), time-resolved reflection high energy electron diffraction (Tr-RHEED) and time-resolved transmission electron diffraction (Tr-TED) were... Show moreThis dissertation presents the recent developments and experiments performed using the third generation femtosecond electron diffractometer in Professor Jianming Cao's group as well as experiments performed using the previous second generation diffractometer now located at Shanghai Jiao Tong University. Two techniques of ultrafast electron diffraction (UED), time-resolved reflection high energy electron diffraction (Tr-RHEED) and time-resolved transmission electron diffraction (Tr-TED) were developed and applied to study the ultrafast lattice dynamics in semiconductor nanostructures. Tr-RHEED provides the ability to directly monitor the thermal transport across an interface inside a semiconductor quantum well (QW) by measuring the temperature evolution of the first few atomic layers. Tr-TED allows for a measurement of the laser-induced ultrafast structural dynamics of 5 nm PbSe quantum dots (QDs) in real time by diffracting through the entire sample thickness. In the first project, the setup of the first Tr-RHEED experiments and the first successful collection of Tr-RHEED data in our laboratory's history is discussed. The ultrafast temperature evolution of the GaAs nanofilm was measured and numerically modeled using the well known heat conduction equation and also a three-temperature model. These models were fit to the experimental data, allowing for the extraction of the thermal boundary conductance (TBC) and providing a method of measuring TBC in epitaxially grown semiconductor heterostructures. Surprisingly, the TBC was found to increase with increasing temperature even for temperatures above the Debye temperature, opening up questions about the exact mechanisms governing heat transfer at interfaces between very similar semiconductor nanoscale materials. In the second project, we directly monitored the lattice dynamics in PbSe quantum dots induced by laser excitation using Tr-TED. The energy relaxation between the carriers and the lattice took place within 10 ps, showing no evidence of any significant phonon bottleneck effect. Meanwhile, the lattice dilation exhibited some unusual features that could not be explained by the available mechanisms of photon-induced acoustic vibrations in semiconductors alone. The heat transport between the QDs and the substrate deviates significantly from Fourier's Law, which furthers studies about the heat transfer under nonequilibrium conditions in nanoscale materials. In addition to the UED projects, femtosecond transient spectroscopy (FTS) experiments were set up and tested on 20 nm gold nanofilms for various optical excitation laser fluences. The experimental data obtained agrees well with many previous published results. The well known two-temperature model (TTM) was used to describe the temperature evolution and the energy redistribution from the electronic to lattice systems. Using similar experimental and data analysis techniques to the ones developed in this dissertation will pave the way for future FTS experiments performed in conjunction to UED experiments to gain a more complete picture of the ultrafast dynamics in carriers and phonons in complex materials. Show less

The understanding of the electronic systems of materials has not been only the essential, but the driving force, behind the progress of technology for over 100 years. This year marks the 60th anniversary of the revolutionary Bardeen-Cooper-Schrieffer, or BCS, theory which described the creation of Cooper pairs from a Fermi liquid ‘normal’ state through a coupling of conduction elections to phonons. Despite this, it wasn’t until the cuprate La2−xBaxCuO4, the first high-temperature... Show moreThe understanding of the electronic systems of materials has not been only the essential, but the driving force, behind the progress of technology for over 100 years. This year marks the 60th anniversary of the revolutionary Bardeen-Cooper-Schrieffer, or BCS, theory which described the creation of Cooper pairs from a Fermi liquid ‘normal’ state through a coupling of conduction elections to phonons. Despite this, it wasn’t until the cuprate La2−xBaxCuO4, the first high-temperature superconductor, was discovered in the late 1980’s [1] that the dream of a room temperature superconductor seemed attainable and the ‘Age of the Superconductor’ began. However, the unique properties for which these high-temperature, unconventional superconductors are prized have also obstructed thorough investigation of the electronic behavior underlying their superconductivity and demanded extremely intense magnetic fields, very low temperatures, and thermodynamic measurements in extreme environments in order to fully characterize their electronic systems. It is, therefore, no small thing to flesh out the phase diagrams of these materials whose exotic electronic properties may eventually lead to faster, more compact devices and new methods of digital computation. Despite the difficulties in collecting usefully data on high-temperature superconductors, a vast body of work has amassed and grown with the increasingly intense magnetic fields available. As a result, quasiparticle mass enhancement near optimal doping was recently observed in two major classes of high-temperature superconductors, cuprates [2] and pnictides [3–5]. Because an effective quasiparticle mass accounts for the interactions between an electron and surrounding particles, it is an experimental indicator of enhanced electronic interactions. Enhancement of the quasiparticle effective mass, or increased electronic interactions, is believed to accompany quantum criticality, and the observation of mass enhancement in two very different classes of high-temperature superconductors makes quantum criticality the most promising candidate for universality across the high-temperature superconductors. The study outlined here is an investigation of the properties of three high-temperature superconductors, La2−xSrxCuO4, YBa2Cu3Oδ , and BaFe2(As1−xPx)2, through specific heat and resistivity measurements at very low temperatures, 1.5 K ≤ T ≤ 20 K, and magnetic fields up to 35 T. Such measurements required the construction of instrumentation specifically designed to deal with these extreme environments, and the low thermodynamic signals which are a signature of the cuprate superconductors. In order to understand the unprecedented data collected, novel analysis techniques based on Volovik phenomenology were developed. The procedures for specific heat measurements and the analysis of the resulting data developed for this study and outlined in the following thesis stand as the model for measurement of the normal state density of states of correlated superconductors. I report the observation of a saturation of the specific heat as a function of applied magnetic field in all three compounds, La2−xSrxCuO4, YBa2Cu3Oδ , and BaFe2(As1−xPx)2, indicating superconductivity has been suppressed and from which an effective mass, or sum of quasiparticle masses can be determined. I report that the onset of the normal state corresponds to the onset of finite resistance in La2−xSrxCuO4 and BaFe2(As1−xPx)2. I report enhancement in the sum of quasiparticle masses with doping in BaFe2(As1−xPx)2 that diverges near the predicted quantum critical point at optimum doping and that the dramatic enhancement evidences an orbital selective coupling to quantum fluctuations when compared to previous studies. Show less

This thesis studied the electronic structure at the Fermi level and the topological character of topological semimetals via torque magnetometry. Torque magnetometry measures the anisotropic magnetization of the sample in a tilted magnetic field. In our measurements, the magnetic field is up to 35 T and the temperature is down to 300 mK. The oscillatory signal of the magnetization is detected, which is the so called "de Hass van Alphen (dHvA)" effect. By using dHvA effect, many important... Show moreThis thesis studied the electronic structure at the Fermi level and the topological character of topological semimetals via torque magnetometry. Torque magnetometry measures the anisotropic magnetization of the sample in a tilted magnetic field. In our measurements, the magnetic field is up to 35 T and the temperature is down to 300 mK. The oscillatory signal of the magnetization is detected, which is the so called "de Hass van Alphen (dHvA)" effect. By using dHvA effect, many important parameters such as the geometry of Fermi surfaces, effective masses, quantum mobilities, Land ́e g factors and Berry's phases. It is especially important for the Berry's phase extraction. It is known that if there is a cyclotron orbit encircling a Dirac node, a non-trivial Berry's phase π can be extracted and a trivial Berry's phase 0 is expected for a conventional parabolic band. In the study of MAl3, we provided a detailed study of the dHvA oscillations and provided a comparison with the calculated band structures. The angular dependence of their Fermi surface cross-sectional areas reveals a remarkably good agreement with our first-principles calculations. dHvA supports the existence of tilted Dirac cones with Dirac type-II nodes located at 100, 230 and 250 meV above the Fermi level EF for VAl3,NbAl3 and TaAl3 respectively, in agreement with the prediction of broken Lorentz invariance in these compounds. However, for all three compounds we find that the cyclotron orbits on their FSs, including an orbit nearly enclosing the Dirac type-II node, yield trivial Berry phases. We showed that if one would like to derive a convincing Berry's phase from quantum oscillations one has to take into account the spin dephasing term in the LK formalism, and the precise location between the cyclotron orbit and the Dirac node. M2Te2X is studied via both torque magnetometry and angle-resolved photoemission spectroscopy (ARPES). Bulk two-dimensional Fermi surfaces are well-described by the dHvA oscillations and first principles calculations. Intriguingly, slab electronic structure calculations predict Dirac-like surface states at different locations within the Brillouin zone, which is consistent with ARPES observations. Show less

Despite over thirty years of research, the origin of high-temperature superconductivity remains unsolved. In these thirty years, the phase diagram for the rst-discovered high-temperature superconductors, the cuprates, has been found to be rather complex and exhibits many different phases such as antiferromagnetism, charge density waves, spin density waves, nematicity, the pseudogap, and of course, superconductivity. Furthermore, several structural instabilities can manifest that affect the... Show moreDespite over thirty years of research, the origin of high-temperature superconductivity remains unsolved. In these thirty years, the phase diagram for the rst-discovered high-temperature superconductors, the cuprates, has been found to be rather complex and exhibits many different phases such as antiferromagnetism, charge density waves, spin density waves, nematicity, the pseudogap, and of course, superconductivity. Furthermore, several structural instabilities can manifest that affect the stability of these phases. In the La-214 cuprates, for example, it is known the concomitant charge and spin orders (or stripe order) are stabilized by a low-temperature tetragonal structure. The stripe order coincides with a suppression of the superconducting critical temperature, leading the conclusion that these phases either compete or are intertwined. Since the stability of the low-temperature tetragonal structure, and therefore stripes, can be controlled by various dopants, the La-214 cuprates can be used to investigate how these orders intertwine. In this thesis, both striped and unstriped La-214 compounds have been investigated to understand the interplay of these various orders: superconductivity, stripes, and structure. In three distinct studies, using various charge transport techniques, the interplay between these orders is shown to lead to interesting and unexpected behavior. The first study reveals static charge order is in fact a fluctuating order pinned by the structure. The second study shows the two-dimensional nature of the superconductivity in the absence of stripe order, which is speculated to decouple CuO2 planes. Finally, the third study reveals the existence of a hidden order of Cooper pairs in the T=0 field-driven superconducting-normal-state transition when stripes are present. The culmination of these distinct studies lead to a better understanding of the physics of cuprates through the interplay of their various orders, and thus the general phase diagram of high-temperature superconductivity. Show less

Date Issued

2018

Identifier

2018_Sp_Baity_fsu_0071E_14433

Format

Thesis

Title

Computational Studies of Equilibrium and Non-Equilibrium Phase Diagrams and Critical Properties of Two Physical and Chemical Model Systems with Both Short-Range and Long-Range Interactions or Reactivities.

In this dissertation, we introduce long-range interactions into one equilibrium model (Ising model) and one non-equilibrium system (Ziff-Gulari-Barshad model), and study their phase diagrams and critical properties. A new approach to do Wang-Landau simulation: macroscopically constrained Wang-Landau, is proposed in connection with the former system. Our macroscopically constrained Wang-Landau method breaks a multidimensional random walk process in phase space into many separate, one... Show moreIn this dissertation, we introduce long-range interactions into one equilibrium model (Ising model) and one non-equilibrium system (Ziff-Gulari-Barshad model), and study their phase diagrams and critical properties. A new approach to do Wang-Landau simulation: macroscopically constrained Wang-Landau, is proposed in connection with the former system. Our macroscopically constrained Wang-Landau method breaks a multidimensional random walk process in phase space into many separate, one-dimensional random walk processes in the energy space. Each of these random walks is constrained to a different value of the macroscopic order parameters. By knowing the distribution of these constrained variables, we can deduce the multi-variable density of states. When the multi-variable density of states for one set of external parameters is obtained, the density of states at any point in the phase diagram can be obtained by simple transformations. After that, all thermodynamic quantities can be obtained. We apply this method to an antiferromagnetic Ising model with a ferromagnetic long-range interaction. The addition of the long-range interaction induces metastable regions in the phase diagram, and a mean-field class critical point emerges for sufficiently strong long-range interaction. We demonstrate how to use the multi-variable density of states obtained to sketch out the complicated phase diagrams for different values of the long-range interaction. We also give free-energy plots, and plots of the distributions of the order parameters of the system for different special points in these phase diagrams. The Ziff-Gulari-Barshad (ZGB) model, a simplified description of the oxidation of carbon monoxide (CO) on a catalyst surface, is widely used to study properties of nonequilibrium phase transitions. Instead of restricting the CO and atomic oxygen (O) to react to form carbon dioxide (CO₂) only when they are adsorbed in close proximity, we consider a modified model that includes an adjustable probability for adsorbed CO and O atoms located far apart on the lattice to react. We employ large-scale Monte Carlo simulations to study the critical properties of this system. We find that the nonequilibrium critical point changes from the two-dimensional Ising universality class to the mean-field universality class upon introducing even a weak long-range reactivity mechanism. Show less

Electrophoresis as an analytical technique has made considerable contributions to the separations and analysis of macromolecules in biology-related research. Pluronic gels, which are composed of orderly packed spherical micelles assembled by tri-block copolymers, have been developed as novel sieving media to separate oligonucleotides, duplex DNA molecules and proteins, providing ease of manipulations due to their thermo-reversibility and higher resolution in comparison with other polymer gels... Show moreElectrophoresis as an analytical technique has made considerable contributions to the separations and analysis of macromolecules in biology-related research. Pluronic gels, which are composed of orderly packed spherical micelles assembled by tri-block copolymers, have been developed as novel sieving media to separate oligonucleotides, duplex DNA molecules and proteins, providing ease of manipulations due to their thermo-reversibility and higher resolution in comparison with other polymer gels. Electrophoretic mobility of short double-stranded DNA molecules in pluronic F127 is reported to have a non-monotonic dependence on DNA length, which is not observed in other polymer-based sieving media or explained by any well-developed theories. In this dissertation, the unusual DNA-length dependence of electrophoretic mobility is experimentally investigated in several different pluronic gels, and the DNA dynamics in pluronic liquid crystals is systematically studied by coarse-grained Brownian dynamics simulations. The crystal structures and micelle dimensions of pluronics P105, P123 and F127 are characterized by atomic force microscopy, small-angle x-ray scattering, small-angle neutron scattering and dynamic light scattering. Two-dimensional gel electrophoresis is performed and the electrophoretic mobility of DNA molecules in the size range of 20-500 bp is measured in pluronics P105, P123 and F127. The unusual DNA length-dependent mobility is consistently obtained in three pluronic gels, where the mobility of very short DNA molecules increases with increasing DNA length, and the mobility of long DNA molecules monotonically decreases with DNA length. Superposed on the rising and falling trends are the subtle oscillations of mobility with DNA length in the intermediate regime. Brownian dynamics simulations are implemented to numerically calculate the DNA mobility in pluronic lattices, by including the short-ranged intra-molecular hydrodynamic interactions, and modeling the interactions between DNA molecules and pluronic micelles via a repulsive force and entanglement effect. The rise, fall and oscillations of mobility with DNA length, as obtained in experimental measurements, are reproduced by the Brownian dynamics simulations, and essential physics that dominates the unusual features of mobility is extracted from the simulations. In addition, electric field-dependent mobility of DNA molecules in pluronic lattices is studied by Brownian dynamics simulations, and the conceptual connection between high-field simulations along specific field directions and low-field experiments in bulk gels is established, and the Brownian dynamic simulations are proven to be an appropriate approach to interpret the DNA electrophoretic dynamics in pluronic matrices. Moreover, electrophoretic mobility of duplex DNA flanked by single-stranded overhangs is measured in pluronic gels, and it is shown that the mobility of DNA with overhangs is higher than the corresponding blunt-ended DNA molecules. Brownian dynamics simulations are carried out, and the enhancement of mobility for DNA with overhangs is captured by the simulations. By integrating numerical simulations with experimental measurements, the fundamental physical quantities and interactions that manipulate the DNA electrophoretic migration in pluronic liquid crystals are revealed. Understanding the unusual DNA length-dependent mobility in pluronic gels potentially provides profound insights in designing and optimizing high-performance sieving matrices for size-based separation purposes. Show less

Single crystals of l-ascorbic acid and partially deuterated l-ascorbic acid were irradiated in the temperature range of 77K to 300K by either x-rays from a 3 Mev Van de Graaff electron accelerator or UV radiation (337.1 nm) from a pulsed nitrogen gas laser having a peak power of 240 KW. Several types of free radicals were produced by the radiation and were studied by X-band and Q-band ESR from 4.2K to 300K. The kinds of radicals produced by the UV and x-irradiation were the same. The relative... Show moreSingle crystals of l-ascorbic acid and partially deuterated l-ascorbic acid were irradiated in the temperature range of 77K to 300K by either x-rays from a 3 Mev Van de Graaff electron accelerator or UV radiation (337.1 nm) from a pulsed nitrogen gas laser having a peak power of 240 KW. Several types of free radicals were produced by the radiation and were studied by X-band and Q-band ESR from 4.2K to 300K. The kinds of radicals produced by the UV and x-irradiation were the same. The relative intensities of the spectral lines of the different types of radicals depend upon both the temperature and the type of radiation. Four radicals were identified: two oxidation products (radicals I and IV), one neutralized anion (radical III), and one adduct (radical II). Radicals I, III, and IV are present at 77K after irradiation at 77K. Upon annealing to 140K, radical I changes to dehydroascorbic acid and proton adduction creates radical II. Upon annealing to 300K, radicals II, III, and IV remain and are stable at 300K for at least several months. All four radicals are carbon based radicals and are located on the furan ring. All hyperfine couplings are to (beta)-protons. No carbon-13 hyperfine couplings were observed. ENDOR was tried at 4.2K, 77K, and 300K after x-irradiation at 77K or 300K. In all cases a matrix ENDOR line was observed, but no hyperfine ENDOR lines were observed. UV bleaching was attempted at 300K on a deuterated crystal which had been x-irradiated previously. The UV laser radiation had no observable effect on the ESR spectra in which radicals II, III, and IV were already present. Show less

Date Issued

1980, 1980

Identifier

AAI8108399, 3084829, FSDT3084829, fsu:74330

Format

Document (PDF)

Title

Dynamical properties of strongly correlated fermionic systems.

Creator

Haas, Stephan Wolfgang., Florida State University

Abstract/Description

The Exact Diagonalization method is a powerful numerical tool to study Quantum Many Body systems on finite clusters. In particular, using this technique one can accurately calculate energy and momentum dependent dynamic correlation functions which are observable in scattering experiments, such as Neutron Scattering, Raman Scattering, and Photoemission Spectroscopy which measures the spectral function of the system. Here we give an outline of the Lanczos method with special emphasis on the... Show moreThe Exact Diagonalization method is a powerful numerical tool to study Quantum Many Body systems on finite clusters. In particular, using this technique one can accurately calculate energy and momentum dependent dynamic correlation functions which are observable in scattering experiments, such as Neutron Scattering, Raman Scattering, and Photoemission Spectroscopy which measures the spectral function of the system. Here we give an outline of the Lanczos method with special emphasis on the evaluation of dynamical quantities., In this thesis, we apply this method to two-dimensional models of strongly correlated electrons which are believed to describe the physics of the recently discovered cuprate high-$\rm T\sb{c}$ compounds. We show that simple models of strongly correlated electrons, such as the Hubbard and the t-J model, can account for some normal state properties of these materials. In particular, the occurance of photoemission bands which are introduced by short-range antiferromagnetic correlations is discussed., The precursor materials of the cuprate superconducters are antiferromagnets. Here, we address the properties of antiferromagnets as they evolve from an insulating to a metallic phase upon doping. We focus on the shape of the Fermi surface at small hole doping and on the influence of long-range Coulomb interactions on the occurance of superconducting and charge density wave phases., We also investigate systems in one spatial dimension where mechanisms similar to the ones in higher dimensions can be studied on larger clusters. However, there are some significant dimension dependent differences, e.g. in contrast to the two-dimensional case, one-dimensional antiferromagnets exhibit a gapped spectrum if the participating spins have integer value. We discuss the physics of these 'Haldane' chains. The calculated spectra for these materials are in excellent agreement with recent Neutron Scattering Experiments., The effect of random exchange interaction in quantum antiferromagnets is also discussed. We show that such interactions do not necessarily induce an exponential decay in the spin correlations. Also, we argue that random exchange interactions can be induced by phononic disorder and might be responsible for the lineshape of Raman spectra observed in the cuprates. Our calculated Raman spectra are in good agreement with recent experiments on various cuprate precursors. Show less

Transfer-matrix and finite-size scaling methods are applied to two different lattice-gas models to elucidate equilibrium and nonequilibrium aspects of first-order phase transitions in systems with interactions of finite range., The first model is derived from the anisotropic (asymmetric) next-nearest-neighbor interaction (ASYNNNI) lattice-gas model which describes the structural phase transitions of the high-temperature superconductor YBa$\sb2$Cu$\sb3$0$\sb{6 + x}$ in terms of second-order... Show moreTransfer-matrix and finite-size scaling methods are applied to two different lattice-gas models to elucidate equilibrium and nonequilibrium aspects of first-order phase transitions in systems with interactions of finite range., The first model is derived from the anisotropic (asymmetric) next-nearest-neighbor interaction (ASYNNNI) lattice-gas model which describes the structural phase transitions of the high-temperature superconductor YBa$\sb2$Cu$\sb3$0$\sb{6 + x}$ in terms of second-order phase transitions in the CuO basal planes. To account for experiments indicating that these transitions might be first-order at low temperatures, we extend the ASYNNNI model to include weak attractive anisotropic interactions between next-nearest-neighbor oxygen chains. Using the conventional transfer-matrix method and finite-size scaling, we calculate the equilibrium phase diagram and find first-order phase transitions and tricritical points at low temperatures for the order-order as well as the order-disorder transitions., The second model is the two-dimensional square-lattice nearest-neighbor Ising ferromagnet. We study the metastability displayed by this model below its critical temperature and in an external field. Applying a constrained-transfer-matrix formalism, we obtain complex-valued constrained free energies. In particular, we study the imaginary part of the constrained free-energy branch that corresponds to the metastable phase. Although droplets are not introduced explicitly, the metastable free energy is obtained in excellent agreement with field-theoretical droplet-model predictions. The finite-size scaling properties are different in the weak-field and intermediate-field regimes, and we identify the corresponding different critical-droplet shapes. Our results extend the region of validity for known results of the field-theoretical droplet model, and they indicate that this transfer-matrix approach provides a nonperturbative numerical continuation of the equilibrium free energy into the metastable phase. Show less

Date Issued

1994, 1994

Identifier

AAI9511343, 3088493, FSDT3088493, fsu:77297

Format

Document (PDF)

Title

NMR IN PARAMAGNETIC NEODYMIUM(III)-BROMIDE AND URANIUM(III)-IODIDE AND INANTIFERROMAGNETIC URANIUM(III)-IODIDE.

COMPUTATIONAL FEASIBILITY OF A VARIATIONAL PRINCIPLE IN AN EXTENDED HILBERT SPACE.

Creator

CHAN, LORENZO CHUA., The Florida State University

Date Issued

1968, 1968

Identifier

AAI6913260, 2985932, FSDT2985932, fsu:70441

Format

Document (PDF)

Title

SUPERCONDUCTING AND NORMAL STATE PROPERTIES OF HEXAGONAL TUNGSTEN BRONZES RUBIDIUM(X)TUNGSTATE, RUBIDIUM(X)CESIUM(Y)TUNGSTATE AND POTASSIUM(X)TUNGSTATE.

Creator

CADWELL, LOUIS HENRY., The Florida State University

Date Issued

1980, 1980

Identifier

AAI8014114, 2989539, FSDT2989539, fsu:74046

Format

Document (PDF)

Title

HIGH-TEMPERATURE SERIES EXPANSION TECHNIQUE FOR SYSTEMS WITH COMPLICATED ENERGY LEVELS.

Creator

JOHNSON, JAMES WRAY., The Florida State University

Abstract/Description

A new approach to the high-temperature series expansion which is applicable to systems with complicated energy level schemes such as magnetic systems with crystal-field anisotropy of arbitrary strength has been formulated. We compare this approach with the original Green's function formulation of Wand and Lee and discuss the advantage of the present technique. We apply the approach developed here to the spin-one hard-axis Heisenberg ferromagnet and obtain the first four terms in the high... Show moreA new approach to the high-temperature series expansion which is applicable to systems with complicated energy level schemes such as magnetic systems with crystal-field anisotropy of arbitrary strength has been formulated. We compare this approach with the original Green's function formulation of Wand and Lee and discuss the advantage of the present technique. We apply the approach developed here to the spin-one hard-axis Heisenberg ferromagnet and obtain the first four terms in the high-temperature series expansion for the free energy, the magnetic susceptibility, and the specific heat. The formula obtained for the free energy of the spin-one hard-axis ferromagnet also describes spin-one systems with rhombic anisotropy and reduces to the spin-one ferromagnet with an easy-axis anisotropy and to the spin-one simple Heisenberg systems by setting the appropriate matrix elements equal to zero. The method can be extended to treat systems with spin greater than one with general crystal field symmetry. The calculation of higher order terms is rendered tractable using the approach developed here. Show less

Fe$\sb3$O$\sb4$ (magnetite) is a ferromagnetic semiconductor while NiO is an antiferromagnetic insulator with a room temperature resistivity at least six orders of magnitude greater than that of Fe$\sb3$O$\sb4.$ Modulated structure films, with equal Fe$\sb3$O$\sb4$ and NiO layer thicknesses, were grown using plasma assisted molecular beam epitaxy to a total film thickness of 3446A, and with modulation wavelengths $\Lambda$ (bilayer thicknesses) ranging from 16A to 1763A. Post growth $\theta$... Show moreFe$\sb3$O$\sb4$ (magnetite) is a ferromagnetic semiconductor while NiO is an antiferromagnetic insulator with a room temperature resistivity at least six orders of magnitude greater than that of Fe$\sb3$O$\sb4.$ Modulated structure films, with equal Fe$\sb3$O$\sb4$ and NiO layer thicknesses, were grown using plasma assisted molecular beam epitaxy to a total film thickness of 3446A, and with modulation wavelengths $\Lambda$ (bilayer thicknesses) ranging from 16A to 1763A. Post growth $\theta$-2$\theta$ x-ray data contain well defined low angle peaks which confirm that the targeted layer thicknesses were accurately achieved. Resistivity has been measured perpendicular to the plane of the film, as a function of modulation wavelength and temperature. A dependence of the resistivity on $\Lambda$ is observed in two sample sets in which the resistivity rapidly increases many orders of magnitude as the modulation wavelength decreases from the bulk $\Lambda\rightarrow\infty$ limit in the vicinity of 600A. This length scale dependent resistivity enhancement of the Fe$\sb3$O$\sb4$/NiO modulated structures cannot be explained by the standard model of interfacial resistance. A qualitative argument is presented for a metal-insulator-metal interfacial charge transfer model which contains both the resistivity enhancement and the observed length scale dependence. Show less

Date Issued

1996, 1996

Identifier

AAI9627216, 3088923, FSDT3088923, fsu:77722

Format

Document (PDF)

Title

Finite-range scaling analysis of criticality and metastability in Ising ferromagnets.

Creator

Gorman, Bryan Michael., Florida State University

Abstract/Description

Analytical and numerical methods are used to study the stationary properties of equilibrium and metastable phases in scalar field theories and model systems with weak, long-range forces (WLRF) by determining the finite-range scaling (FRS) of the free energy F and its analytic continuation $\tilde F$ into the metastable phase. The scaling properties of d-dimensional $\phi\sp n$ scalar field theories are derived, and two special cases are used to study equilibrium and non-equilibrium critical... Show moreAnalytical and numerical methods are used to study the stationary properties of equilibrium and metastable phases in scalar field theories and model systems with weak, long-range forces (WLRF) by determining the finite-range scaling (FRS) of the free energy F and its analytic continuation $\tilde F$ into the metastable phase. The scaling properties of d-dimensional $\phi\sp n$ scalar field theories are derived, and two special cases are used to study equilibrium and non-equilibrium critical phenomena in WLRF systems. A criterion of critical equivalence is identified, relating the FRS of WLRF systems to the finite-size scaling of hypercylindrical systems above the upper critical dimension. A method of analytically continuing the equilibrium free energy into the metastable phase is generalized for systems exhibiting multiple metastable phases, and new scaling results for $\tilde F$ are found near classical spinodals, including exact results for $d=1.$ An analytic continuation of the free energy is performed numerically on two hypercylindrical systems and compared to analytic expansions for equivalent field theories. Transfer-matrix (TM) finite-size scaling confirms the critical exponents for $d=1$ WLRF systems. For metastable phases, a constrained-transfer-matrix (CTM) method is applied, in which one obtains a "constrained" free-energy density computed directly from the TM. Monte Carlo simulation is performed to obtain decay rates directly, which are compared with both the CTM results and the analytic continuation using Langer's proportionality relation. Show less

Date Issued

1994, 1994

Identifier

AAI9516731, 3088545, FSDT3088545, fsu:77347

Format

Document (PDF)

Title

A numerical investigation of the finite-size scaling properties of superfluid helium.

Creator

Schultka, Norbert., Florida State University

Abstract/Description

We numerically investigate the finite-size scaling properties of the superfluid density and of the specific heat of superfluid $\sp4He$ confined in cubic and film geometries by using the $x - y$ model and the Cluster Monte-Carlo method. We show that the superfluid density and the specific heat of $\sp4He$, confined in a cubic geometry, scale with respect to the linear length of the system according to finite-size scaling theory, and we derive the temperature dependence of these quantities in... Show moreWe numerically investigate the finite-size scaling properties of the superfluid density and of the specific heat of superfluid $\sp4He$ confined in cubic and film geometries by using the $x - y$ model and the Cluster Monte-Carlo method. We show that the superfluid density and the specific heat of $\sp4He$, confined in a cubic geometry, scale with respect to the linear length of the system according to finite-size scaling theory, and we derive the temperature dependence of these quantities in the bulk limit by extrapolating the values of the superfluid density and the specific heat obtained for finite lattices to the values corresponding to a lattice of infinite extent. In the case of the film geometry, $\sp4He$ exhibits a Kosterlitz-Thouless phase transition at the thickness-dependent critical temperature, i.e., close to this temperature superfluid helium behaves effectively two-dimensional. We show that the boundary conditions imposed in the top and bottom layers of the film strongly influence the shape of the universal scaling functions of the superfluid density and the specific heat with respect to the film thickness. We always compare our results to the experiments. Show less

Date Issued

1995, 1995

Identifier

AAI9527939, 3088650, FSDT3088650, fsu:77452

Format

Document (PDF)

Title

A study of the partition functions of two statistical systems.

Creator

Carter, Paul Alan., Florida State University

Abstract/Description

The numerical calculation of the partition function of the 3d Ising model and the 2d XY model are performed using the newly developed Spectral Density method. The performance and utility of the method are also shown. The finite size scaling of the complex zeros of the partition function is used to estimate the critical exponent $\nu$ for the two models. Good estimates of $\nu$ exist for the 3d Ising model and are found to agree with the estimates calculated here. Recently, there has been... Show moreThe numerical calculation of the partition function of the 3d Ising model and the 2d XY model are performed using the newly developed Spectral Density method. The performance and utility of the method are also shown. The finite size scaling of the complex zeros of the partition function is used to estimate the critical exponent $\nu$ for the two models. Good estimates of $\nu$ exist for the 3d Ising model and are found to agree with the estimates calculated here. Recently, there has been debate over whether the XY model has an infinite order phase transition as previously believed or a finite order transition. The critical exponent $\nu$ is sensitive to this question. Unlike a finite order transition, it is impossible to define $\nu$ for an infinite order transition. It is shown that if estimates of $\nu$ are measured for an infinite order transition, the estimates will diverge in the thermodynamic limit. The nature of the XY model's phase transition is explored by finding estimates of $\nu$ and the behavior of the derivatives of the free energy; however, the results are inconclusive. Show less

The alkali halide surfaces were the first for which full surface phonon dispersion curves were measured. Most of the early work was mainly concerned with the low-energy modes for the lighter alkali halides, such as LiF and NaF, and a full survey of a more representative sampling of these crystals was never made. A recent theoretical calculation has also suggested that relaxation effects might provide interesting features on the surface phonon modes especially for the optical modes of the... Show moreThe alkali halide surfaces were the first for which full surface phonon dispersion curves were measured. Most of the early work was mainly concerned with the low-energy modes for the lighter alkali halides, such as LiF and NaF, and a full survey of a more representative sampling of these crystals was never made. A recent theoretical calculation has also suggested that relaxation effects might provide interesting features on the surface phonon modes especially for the optical modes of the heavier compounds. The work of this dissertation is an effort to measure the surface dynamics of other, usually heavier, alkali halide crystals. This work describes measurements on the surfaces of RbCl(001), RbBr(001), KBr(001), and NaI(001) with the inelastic He atom scattering technique. The energy gain or loss and the momentum change is measured by the time of flight method. For the four crystals, the surface phonon dispersion curves are obtained along both high symmetry directions. The general agreement between the theory and the experimental results is very good. Some new features of the surface vibration have been found. First, the optical mode, crossing resonance and acoustic modes were clearly seen from the KBr surface over the entire zone. The strong interaction coupling between the He and the optical mode is rather unique. Secondly, crossing resonances (S$\sb8$ mode) were observed for the KBr, RbBr and NaI surfaces. This observation suggests that this mode is a general feature of all the alkali halide surfaces. The origin of this mode might be due to the perpendicular polarized bulk acoustic motion. Thirdly, NaI results show that bound-state resonance effects could strongly enhance the bulk mode especially when there is a high density of phonon states. Finally, there is no experimental evidence supporting any surface relaxation effects. In fact, the experimental, results seem to agree better with the unrelaxed calculations. In addition to the inelastic scattering measurements, we also did angular distribution, low pressure He beam scattering and a temperature-dependent dynamical study. Information such as the surface corrugation, He-surface potential, bound state energies, the surface Debye-Waller temperature and a measure of multi-phonon contributions can be obtained from these experimental results. Show less